Uncovering the unusual effect of halogenation on crystal packing in an azzaacee-based electron transporting material

Abstract Due to strong interaction with perovskite, azzaacee-based derivative TDTP is promising electron-transporting material (ETM) in the inverted perovskite solar cell. However, the low electron mobility of TDTP seriously limits the photoelectric conversion efficiency (PCE) of the device. Herein, three TDTP-type material molecules, FT, ClT and BrT, are designed based on the paternal TDTP to boost the electron mobility. The results demonstrate that the introduction of halogen (electron-withdrawing group) improves the electron mobility of the materials due to the enhanced intermolecular π-π stacking. Unexpectedly, ClT with chlorine substitution violates the electronegative order and shows the highest electron mobility, which results from the head-to-head stacking manner only in ClT crystal. This special stacking produces stronger electron coupling between molecules, thus facilitating carrier transport. This work provides theoretical guidance for the design of ETMs with excellent electron mobility by deeply understanding the relationship between the charge transporting properties of ETMs and their molecular/crystal structures.